Parabolic trough type solar concentrators utilize a fluid filled receiver tube. The fluid within the receiver tube operates as a heat transfer fluid. Often, the heat transfer fluid is a synthetic oil having a very high thermal capacity. Recent research has indicated that the use of a molten salt as a heat transfer fluid can provide various advantages. However, a molten salt can freeze at higher temperatures than other heat transfer fluids. In a parabolic trough solar concentrator power plant utilizing a heat transfer fluid that freezes at or above ambient temperatures, precautions must be taken to prevent and recover from the freezing of the heat transfer fluid in the pipes. In particular, at night or when the sun is obscured by clouds, the temperature of the heat transfer fluid can drop below its freezing point. For example, certain heat transfer fluids, including some molten salts, need to be maintained at a temperature of at least 160° C. in order to prevent the fluid from freezing.
The receiver tube of a parabolic trough solar concentrator is typically vacuum insulated within a glass envelope. Because of this unique construction, conventional heating techniques, such as heat tracing or resistive wire, are not practical. An alternate technique for heating the receiver tube is an impedance or skin effect heating system. In an impedance heating system, current is fed through metal cross-section of the fluid filled pipe or tube. However, conventional impedance heating systems can be difficult or impractical to implement in connection with a parabolic trough solar concentrator, and can suffer from a relatively short service life. For example, the terminal plate used to provide an electrical interconnection to the receiver tube is usually welded parallel to the length of the receiver tube and is just large enough to make a connection to the cable connecting the terminal plate to the power supply. In a parabolic trough solar concentrator, this type of connection would be impractical due to space constraints and the large amount of sunlight seen along the length of the receiver tube. In addition, the conventional terminal plate design would provide a limited contact area with the receiver tube, and complicates insulating the receiver tube. Conventional techniques would also suffer from a shortened service life due to routing the cable on or in the ground beneath the solar concentrator, which can lead to fatigue related failure of the cable as a result of the movement of the parabolic trough to follow the sun.